Testing prompted by an Associated Press story that revealed trace amounts of pharmaceuticals in drinking water supplies has shown that more Americans are affected by the problem than previously thought — at least 46 million.

That’s up from 41 million people reported by the AP in March as part of an investigation into the presence of pharmaceuticals in the nation’s waterways.

The AP stories prompted federal and local legislative hearings, brought about calls for mandatory testing and disclosure, and led officials in at least 27 additional metropolitan areas to analyze their drinking water. Positive tests were reported in 17 cases, including Reno, Nev., Savannah, Ga., Colorado Springs, Colo., and Huntsville, Ala. Results are pending in three others.

The test results, added to data from communities and water utilities that bowed to pressure to disclose earlier test results, produce the new total of Americans known to be exposed to drug-contaminated drinking water supplies.

The overwhelming majority of U.S. cities have not tested drinking water while eight cities — including Boston, Phoenix and Seattle — were relieved that tests showed no detections.

“We didn’t think we’d find anything because our water comes from a pristine source, but after the AP stories we wanted to make sure and reassure our customers,” said Andy Ryan, spokesman for Seattle Public Utilities.

The substances detected in the latest tests mirrored those cited in the earlier AP report.

Chicago, for example, found a cholesterol medication and a nicotine derivative. Many cities found the anti-convulsant carbamazepine. Officials in one of those communities, Colorado Springs, say they detected five pharmaceuticals in all, including a tranquilizer and a hormone.

“This is obviously an emerging issue and after the AP stories came out we felt it was the responsible thing for us to do, as a utility, to find out where we stand. We believe that at these levels, based on current science, that the water is completely safe for our customers,” said Colorado Springs spokesman Steve Berry. “We don’t want to create unnecessary alarm, but at the same time we have a responsibility as a municipal utility to communicate with our customers and let them know.”

Fargo’s water director, Bruce Grubb, said the concentrations of three drugs detected there were so incredibly minute — parts per trillion — that he sent them to the local health officer to figure out how to interpret the information for the community.

“We plan to put this into some kind of context other than just scientific nomenclature, so folks can get some level of understanding about what it means,” said Grubb.

The drug residues detected in water supplies are generally flushed into sewers and waterways through human excretion. Many of the pharmaceuticals are known to slip through sewage and drinking water treatment plants.

While the comprehensive risks are still unclear, researchers are finding evidence that even extremely diluted concentrations of pharmaceutical residues harm fish, frogs and other aquatic species in the wild and impair the workings of human cells in the laboratory.

And while the new survey expands the known extent of the problem, the overwhelming majority of U.S. communities have yet to test, including the single largest water provider in the country, New York City’s Department of Environmental Protection, which delivers water to 9 million people.